Abstract

We review the tectonic evolution of the SW Pacific east of Australia from ca 120 Ma until the present. Akey factor that developed early in this interval and played a major role in the subsequent geodynamichistory of this region was the calving off from eastern Australia of several elongate microcontinental ribbons,including the Lord Howe Rise and Norfolk-New Caledonia Ridge. These microcontinental ribbonswere isolated from Australia and from each other during a protracted extension episode from ca 120 to52 Ma, with oceanic crust accretion occurring from 85 to 52 Ma and producing the Tasman Sea andthe South Loyalty Basin. Generation of these microcontinental ribbons and intervening basins wasassisted by emplacement of a major mantle plume at 100 Ma beneath the southern part of the LordHowe Rise, which in turn contributed to rapid and efficient eastward trench rollback. A major changein Pacific plate motion at ca 55 Ma initiated east-directed subduction along the recently extinct spreadingcentre in the South Loyalty Basin, generating boninitic lithosphere along probably more than 1000km of plate boundary in this region, and growth of the Loyalty-Entrecasteaux arc. Continued subductionof South Loyalty Basin crust led to the arrival at about 38 Ma of the 70-60 million years old westernvolcanic passive margin of the Norfolk Ridge at the trench, and west-directed emplacement of theNew Caledonia ophiolite. Lowermost allochthons of this ophiolite are Maastrichtian and Paleocene rifttholeiites derived from the underthrusting passive margin. Higher allochthonous sheets include a poorlyexposed boninitic lava slice, which itself was overridden by the massive ultramafic sheets that coverlarge parts of New Caledonia and are derived from the colliding forearc of the Loyalty-Entrecasteauxarc. Post-collisional extensional tectonism exhumed the underthrust passive margin, parts of which haveblueschist and eclogite facies metamorphic assemblages. Following locking of this subduction zone at38-34 Ma, subduction jumped eastward, to form a new west-dipping subduction zone above whichformed the Vitiaz arc, that contained elements which today are located in the Tongan, Fijian, Vanuatuand Solomons arcs. Several episodes of arc splitting fragmented the Vitiaz arc and produced first theSouth Fiji Basin (31-25 Ma) and later (10 Ma to present) the North Fiji Basin. Collision of the Ontong JavaPlateau, a large igneous province, with the Solomons section of the Vitiaz arc resulted in a reversal ofsubduction polarity, and growth of the Vanuatu arc on clockwise-rotating, older Vitiaz arc and South FijiBasin crust. Continued rollback of the trench fronting the Tongan arc since 6 Ma has split this arc andproduced the Lau Basin-Havre Trough.This southwest Pacific style of crustal growth above a rolling-back slab is applied to the 600-220 Matectonic development of the Tasman Fold Belt System in southeastern Australia, and explains keyaspects of the geological evolution of eastern Australia. In particular, collision between a plume-triggered600 Ma volcanic passive margin and a 510-515 Ma boninitic forearc of an intra-oceanic arc hadthe same relative orientation and geological effects as that which produced New Caledonia. A newsubduction system formed probably at least several hundred kilometres east of the collision zone andproduced the Macquarie arc, in which the oldest lavas were erupted ca 480 Ma. Continued slab rollbackinduced regional extension and the growth of narrow linear troughs in the Macquarie arc, whichpersisted until terminal deformation of this fold belt in the late-Middle to Late Devonian. A similar patternof tectonic development generated the New England Fold Belt between the Late Devonian and LateTriassic. Parts of the New England Fold Belt have been broken from Australia and moved oceanward tolocations in New Zealand, and on the Lord Howe Rise and Norfolk-New Caledonia Rise, during the post-120 Ma breakup. Given that the Tasman Fold Belt System grew between 600 and 220 Ma by crustalaccretion like the southwest Pacific since 120 Ma, facing the open Pacific Ocean, we question whetherthe eastern (Australia-Antarctica) part of the Neoproterozoic Rodinian supercontinent was joined toLaurentia.